1. Field of the Invention
Embodiments of the invention relate to the formation of semiconductor circuitry and, more particularly, to the formation and fabrication of access devices for use in semiconductor devices.
2. State of the Art
The formation of access devices is well known and fabrication processes for creating such devices have been developed and perfected. However, as feature sizes in semiconductor devices decrease, new methods for fabricating access devices are being developed.
In many conventional access devices, such as in dynamic random access memory (DRAM) memory circuits, metal-oxide-semiconductor field-effect transistors (MOSFETs) or the like have been used to form the periphery transistor gates and access transistor gates of the devices. However, as the feature sizes of access devices are reduced, new, scalable, gate structures are desired.
Alternative gate structures that will be scalable over the next few years have been developed for use with access device technology. Two such structures include recessed access device (RAD) gate structures and fin-shaped field effect transistors, which are also known as FinFET gate structures. One advantage of the RAD structures and FinFET structures is that the fabrication processes used to form such structures are compatible with conventional MOSFET fabrication processes. Another advantage of using RAD structures and FinFET structures with access devices is that the RAD structures and FinFET structures are scalable to the smaller feature sizes quickly becoming integrated with current integrated circuit fabrication technologies.
In conventional access devices, the periphery transistors and access transistors are fabricated from MOSFET type gate transistors. The periphery transistors and access transistors are generally fabricated simultaneously and the doping of the transistor structures has been predominantly N-type doping. For example, the preferred doping patterns for access device transistors includes the formation of N+ doped polysilicon for access transistor gates, N+ doped polysilicon for periphery NMOS transistor gates, and N+ doped polysilicon for periphery PMOS transistor gates. In some instances, P+ doped polysilicon has been used for the periphery PMOS transistor gates.
While N-type doping is conventionally used to form access transistors, it would be beneficial to provide access gate transistors having P-type doping. In addition, the simultaneous formation and doping of gate transistors in access device fabrication is sufficient for the formation of N-type doped gate transistors, however, it is not ideal for the formation of access gate transistors having P-type doping. Therefore, it is desirable to develop fabrication methods for forming access transistors having a P-type doping or a P-type workfunction.